AE006-11
The Nature of Needle Activity as seen in VHF

Wednesday, 9 December 2020: 18:04
Virtual
Brian Hare1, Olaf Scholten1, Joseph R Dwyer2, Christopher Francis Francis Sterpka3 and LOFAR Cosmic Ray Key Science Project, (1)University of Groningen, Groningen, Netherlands, (2)University of New Hampshire, Department of Physics and Astronomy and the Space Science Center, Durham, NH, United States, (3)University of New Hampshire, Department of Physics and Space Science Center, Durham, NH, United States
Abstract:
Hare et al. (2019) showed a previously unreported phenomena, termed needles, which occur along positive leaders and are responsible for the majority of VHF emission from positive leaders, which was confirmed by Pu and Cummer (2019).

Understanding these needles is a significant challenge since they tend to emit VHF on very regular intervals (about every 5 ms), termed a twinkle, and each twinkle seems to be negative breakdown that propagates away from the positively charged leader.

For our observations we use the LOFAR radio telescope to map VHF source locations with high temporal resolution and meter-scale precision in full 3D. We show that propagation speeds of discharges inside of needles vary from 105 m/s up to and over 107 m/s. The more slowly propagating needle twinkles tend to have high-density VHF emission that occurs in bursts, very similar to stepped leaders, while the faster needle twinkles emit significantly fewer VHF pulses. Thus different needle twinkles exhibit a range of propagation mechanisms, probably dependent on the temperature. We will also show that negative leaders and return strokes can quench needle activity, which implies that there is some kind of active connection between the positive and negative leaders during needle activity, and that needles compete with negative leaders and return strokes for negative charge. Finally, we will show needles that are spatially close to each other twinkle at different rates, implying that nearby needle activity does not correlate together.

These observations point to the possibility that needles are conducting structures that mostly remain conducting in-between twinkles. For some reason, needles develop a more-positive potential relative to the leader until there is breakdown between the needle and positive leader channel, producing a twinkle. However, it is not at all clear why this potential difference develops or why needles seem to disconnect from the main positive leader channel at such a regular rate.

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